The scale on a map is 7cm : 1km. If two cities are 42cm apart on the map, what is the actual distance in km between the two cities?
Explanation: The scale means that every 7cm on the map corresponds to an actual distance of 1km. A distance of 42cm on the map is the same as 6 $\cdot$ 7cm. The actual distance between the two cities is 6 $\cdot$ 1km, or 6km.